In mobile vehicles the input gears are incrementally actuated with the aid of hydraulic motors. To be able to use pumps having small flow rates at high speeds, it is desirable to disconnect individual hydraulic motors, that is, decouple them from the power train, and reconnect them when necessary. In the disconnected state the hydraulic motors require no oil flow from the pump. The efficiency of the input can thereby be increased, since less moving parts remain in the power train in this disconnected state. The hydraulic motor(s) still remaining in the powertrain take over after disconnection of one hydraulic motor at high speeds and low torque. If the hydraulic motors used are high-speed axial piston motors, the output shaft thereof is connected by a coupling with the power train.
If the hydraulic motors used are externally supported low-speed radial piston motors, their pistons are pressed with rollers into the appertaining cylinders when in the disconnected state.
A hydraulic driving device for a vehicle which in particular is driven by a Diesel engine and has a hydraulic pump actuated by said Diesel engine has been proposed in the applicant's DE-A 196 24 534.6 wherein a hydraulic motor is driven by a hydraulic pump with a variable volume. The pistons are movable along a cam disc situated in the hydraulic motor, there being provided an inner distributor for pressing the pistons in the cylinders of the hydraulic motor. Said distributor is designed so that a definite number of pistons are connected with a first high/low pressure circuit and a definite number of pistons are connected with two separate high/low pressure circuits, both high/low pressure circuits being connected with a tank for the discharge of pressure fluid.
By virtue of the two high-low pressure circuits it is possible to connect with a tank the high-pressure side of the hydraulic pump or the high-pressure side of the respective hydraulic motor whereby disconnected pistons in the hydraulic motor are no longer loaded with low pressure but connected to the almost pressureless tank. During rotation of the hydraulic motor the cams or treads of the cam disc press the disconnected pistons in their cylinders inwardly in direction of the axis of rotation. The pistons thus remain in their inserted position in the cylinders and cause no loss of efficiency, since no hydraulic fluid flows therein and the mechanical friction loss is also eliminated, since the pistons no longer have any contact with the cam disc in this disconnected state.
If during travel of the vehicle, a hydraulic motor, which prior to the disconnection, had an operational displacement above zero, is disconnected, the pump cannot, within the brief period of disconnection, adjust itself to the lower oil volume then needed and the driver will notice a disengagement jerk. When the disconnected hydraulic motor is again connected, the pump cannot adjust itself to the changed situation during the brief switching time, that is, the pump delivers too little oil; the driver will experience an unpleasant engagement jerk.
Therefore, in the applicant's DE-A 196 24 531.1 was already proposed in relation to a hydraulic power device for a vehicle which has an adjustable, motor-driven hydraulic pump in which at least one input unit is actuated, to develop it in the sense of providing at least two hydraulic motors switchable independently of each other and having displacements which can be respectively controlled by means of a hydraulic motor shut-off valve and a displacement shut-off valve interconnected between the latter and the hydraulic motor, wherein during the connection and disconnection of pistons of at least one hydraulic motor the hydraulic pump can be separated from the input unit by a synchronizing valve disposed between the hydraulic pump and the shut-off valves of the hydraulic motor. Engagement jerks during connection and disconnection can be prevented by providing this synchronizing valve. In this arrangement there is, however, an interruption of drive torque during connection and disconnection of the hydraulic motor.
The problem to be solved by this invention is to prevent the interruption of drive torque during connection and disconnection of a hydraulic motor to and from the power train of mobile vehicles by providing a method and a device having an increased efficiency relative to the engagement jerks produced by connection and disconnection.